The invention relates to an apparatus for adding a first fluid into a second fluid that flows in a conduit. The apparatus has a tube shaped element having a flow-through channel for the second fluid that has a substantially constant cross sectional area. The apparatus has one or many chambers that extend about at least a major portion of the circumference of the flow-through channel along at least a portion of the lengthwise extension of the channel. The apparatus has a connection for inputting the first fluid into the chamber from a pressure source wherein a series of through holes are disposed in the tubular shaped element in the area of the one or many chambers. The first fluid can be conveyed through the holes into the second fluid that flows through the flow-through channel under the influence of the pressure difference between the chamber and the flow-through channel.
Apparatuses of the type mentioned above are known through, for example, SE 468 341 and SE 502 393. The apparatus that is described in SE 502 393 is mostly used as a mixer in bleaching facilities in the cellulose industry for mixing in steam into a pulp suspension to raise its temperature to a level that is desirable for a certain reaction to take place at the desired rate in the subsequent bleaching step. The apparatus can provide good mixing of steam into the suspension, but it is difficult to control the amount of steam that is mixed to control the temperature without reducing the effectiveness of the mixing at the same time. The steam mixing is conventionally controlled by using valves on the steam conduit to the chamber. When the steam inflow is reduced to reduce the steam addition, the pressure in the chamber is also reduced and therefore the pressure difference between the inside of the chamber and the pulp suspension in the conduit. This leads, in turn, to reducing the speed of the steam when it enters the flow through conduit of the pulp and therefore the penetration of the steam into the pulp suspension is also reduced.
Typical for SE 468 341 is that the flow-through channel is shaped as a narrow, ring shaped passage for the second fluid which is believed to promote the mixing effect. Without taking a standpoint whether this is correct or not, or if it applies under certain conditions, it can be concluded that the construction has certain practical problems. This should have something to do with the first fluid, when it with a high speed is injected into the second fluid that flows through the narrow channel, interacts with the constricted body that is arranged in the flow-through conduit and that, probably due to resonance phenomena, causes a serious vibration in the apparatus.
Another drawback of the known apparatuses is that the holes for adding steam become clogged up after some use. This is probably mostly due to scaling, i.e. deposits that are built up on the inner walls of the holes, but also the shape of the inlet holes have some effect. Scaling means that a ceramic-like hard coating is formed downstream of the inlet holes of the steam and the coating requires a substantial amount of mechanical treatment to be removed. Alternatively, a chemical treatment, such as acid washing, can be used to remove the deposits. This requires regular intervals of service wherein the pulp flow is shut off and the deposits are chipped away. Scaling takes place at a higher temperature especially when mixing in steam into pulp from hardwood. The deposits mainly consist of calcium carbonate. As may be realized, the clogging of the openings makes it problematic to add a desired amount of steam and there will be problems related to supplying a controlled amount of steam.
Yet another drawback with the known apparatuses is that an uneven temperature distribution in the pulp suspension along the steam conduit is obtained. Sometimes a temperature differential of about 10xc2x0 C. has been measured between the upper and lower point in a cross section in the subsequent conduits. It is obviously a great drawback to have great temperature differential when working with bleaching chemicals that are often very temperature sensitive, such as hydroperoxide. Furthemore, known apparatuses are relative heavy. Because the material is normally stainless steel, and when the apparatuses also are relatively difficult to manufacture, the total cost of the apparatuses is correspondingly high.
The object of the invention is to provide an apparatus that does not have the above mentioned limitations and drawbacks.
More particularly, the invention relates to an apparatus that provides a good mixing of the first medium into the second medium while providing a good heat distribution of the subsequent conduits, that is, that there are very small temperature differences in an arbitrary selected cross section of the subsequent conduit.
Another positive effect of the apparatus is that is causes relative insignificant vibrations.
Another object is that the need for maintenance is minimized.
Yet another object is that the mixer can be used in very carbonate or calcium rich environments.
These and other objects can be achieved of the apparatus that is described in the appended patent claims. Other characteristics and aspects and advantages of the invention are described in the description below of a preferred embodiment.